Various types of barium titanate-based semiconductor ceramic compositions containing BaTiO3 as a basic composition are known as materials that exhibit positive temperature coefficient (PTC) characteristics. These barium titanate-based semiconductor ceramic compositions have a Curie temperature of about 125° C. PTC heaters composed of such a composition are used at high temperatures, and thus it is necessary that the Curie temperature of the barium titanate-based semiconductor ceramic composition be higher than 125° C. Consequently, in order to increase the Curie temperature, lead titanate (PbTiO3) is added to BaTiO3, thereby forming a solid-solution composition.
However, PbTiO3 contains lead, which is an environmental impact substance. Accordingly, lead-free barium titanate-based semiconductor ceramic compositions having a high Curie temperature have been proposed.
For example, in order to prevent a decrease in a temperature coefficient of resistance due to replacement with lead (Pb) in a barium titanate-based semiconductor ceramic composition to solve a problem of a degradation of withstand voltage characteristics, Japanese Unexamined Patent Application Publication No. 56-169301 (Patent Document 1) discloses a method of producing a barium titanate-based semiconductor ceramic composition including the steps of adding at least one of niobium (Nb), tantalum (Ta), and a rare earth element to a composition having a composition formula of Ba1-2x(BiNa)xTiO3 in which some of barium (Ba) of BaTiO3 is replaced with bismuth (Bi)-sodium (Na) and x satisfies the relationship 0<x≦0.15, firing the resulting mixture in nitrogen, and performing a heat treatment in an oxidizing atmosphere.
In addition, Japanese Unexamined Patent Application Publication No. 2005-255493 (Patent Document 2) proposes a composition for decreasing an electrical resistivity at room temperature in the barium titanate-based semiconductor ceramic composition disclosed in the Patent Document 1. The BaTiO3-based semiconductor ceramic composition proposed in this patent application is a composition in which some of the Ba is replaced with element A1 (one or more of Na, potassium (K), and lithium (Li)) and element A2 (Bi), and some of the Ba is further replaced with a predetermined amount of element Q (one or more of lanthanum (La), dysprosium (Dy), europium (Eu), and gadolinium (Gd)).